The present invention relates to the field of sensors and in particular to sensors with multiplex data output.
Sensors are generally located at the place of the quantity to be measured. This either is required by the measuring principle itself or serves to keep measurement errors and uncertainties to a minimum. In the sensor, the measured quantities, such as temperature, magnetic field, pressure, force, flow rate, filling level, etc., are converted into physical signals which are then fed to the receiving device. As a rule, a conversion into electric signals takes place in the sensor, which signals are easy to generate, transmit, and receive, particularly if the receiver is a processor having appropriate interfaces. The signals to be transmitted can be analog or digital signals, depending on the application. Digital signals have the advantage of being less susceptible than analog signals to interference on the transmission path, but the price paid for this is increased complexity at the transmitting and receiving ends as well as on the transmission path. On the other hand, digital signals frequently fit better into the “signal landscape” of the associated processors, because the signal processing of the latter is substantially digital as well.
To avoid parallel data lines on the transmission path and corresponding parallel connections at the sensor and receiver ends, the data are often transmitted serially. Transmission is effected as a continuous data stream or as data packets separated in time. In the simplest form, the individual bits of the data are encoded by two easily distinguishable logical states and transmitted. There are plenty of known methods, the most widely known being pulse-code modulation (PCM) and pulse-width modulation (PWM), which are both binary modulation methods. Whether a carrier modulation is added does not alter this basically binary modulation scheme.
In the case of longer data words, one disadvantage of serial data transmission is the time needed for transmission, because the transmission rate is relatively slow. Long signal lines may round the pulse edges, reliable detection requires a significantly reduced data rate in comparison with the processor clock rate. As a rule, at least the associated data input of the receiver is blocked for other data during this time; in the worst case, the blocking extends to further portions of the processor, which then does not permit an interrupt, for example.
Another possibility for fast transmission of data is to reconvert the data prior to transmission into an analog signal with discrete values using a digital-to-analog converter, and to transmit this signal. This corresponds to parallel data transmission. At the receiver end, the data can then be recovered from the individual signal ranges by an analog-to-digital converter. At first sight this looks complicated, for the obvious thing to do would be to transmit the sensor's original analog output signal. If, however, processing of the sensor signal, e.g., filtering, interpolation, compensation, level adaptation, equalization, etc., takes place in the sensor, this is accomplished much more easily at the digital level, because then the associated parameters and program steps are retrievable from digital memories and the digital processing is performed in on-chip computing devices. Problems are encountered with this transmission method in the case of high-resolution sensor output signals, because then the interference variables on the transmission path are comparable to or even greater than the step width of the available signal grid.
There is a need for a system and method which permits fast and in particular reliable data transmission between the sensor and receiver even if a high resolution sensor is used.